The objective of this workstatement is to investigate the efficacy of a variety of potential chemopreventive agents in that MNU-induced rat mammary tumor model. This model which entails injection of MNU into 50 day old rats results in tumors with many of the same properties (histopathologic, endocrinologic, response to known therapeutic and chemopreventive agents) that are typical of the human disease. This model will e used to examine a number of potential chemopreventive agents including aromatase inhibitors, differentiating agents, specific inhibitors of prostaglandin synthesis, etc. Agents which are highly effective in these models will be further investigated with respect to their biologic and toxicologic properties and advanced into clinical testing if these initial results appear promising.